Molecular systems, where structure, reactivity and biological activity are “switchable” via an externally controlled factor, create new opportunities for the design of drug delivery systems, optical sensors, molecular switches/logic gate mimics, and a variety of new materials. See, e.g., Choi, H. S., Huh, K. M., Ooya, T., Yui, N. pH- and Thermosensitive Supramolecular Assembling System: Rapidly Responsive Properties of β-Cyclodextrin-Conjugated Poly(ε-lysine). J. Am. Chem. Soc. 125, 6350-6351, (2003); Fabbrizzi, L., Gatti, F., Pallavicini, P., Parodi, L. An ‘off-on-off’ Fluorescent sensor for pH based on ligand-proton and ligand-metal-proton interactions. New J. Chem. 22, 1403-1407, (1998); de Silva, A. P., Gunaratne, H. Q. N., McCoy, C. P. Direct visual indication of pH windows: ‘off-on-off’ fluorescent PET (photoinduced electron transfer) sensors/switches. Chem. Commun., 2399-2400. (1996); Saha, S., Stoddart, J. F. Photo-driven molecular devices. Chem. Soc. Rev. 36, 77-92, (2007);Silvi, S., Arduini, A., Pochini, A., Secchi, A., Tomasulo, M., Raymo, F. M., Baroncini, M., Credi, A. A Simple Molecular Machine Operated by Photoinduced Proton Transfer. J. Am. Chem. Soc. 129, 13378-13379, (2007); and Xue, C., Mirkin, C. A. pH-Switchable Silver Nanoprism Growth Pathways. Angew. Chem. Int. Ed. 46, 2036-2038, (2007). Out of the many external stimuli, pH-gated “switching” is especially useful for addressing biochemical and environmental processes which depend on the acidity of the medium.
In particular, the relatively acidic extracellular environment of solid tumors lends itself for the design of tumor-specific pH-activated chemical agents. See, e.g., Adams, D. J., Dewhirst, M. W., Flowers, J. L., Gamcsik, M. P., Colvin, O. M., Manikumar, G., Wani, M. C., Wall, M. E. Camptotheis analogues with enhanced antitumor activity in acidic pH. Cancer Chemother. Pharmacol. 46, 263-271, (2000); Gabr, A., Kuin, A., Aalders, M., El-Gawly, H., Smets, L. A. Cellular Pharmacokinetics and Cytotoxicity of Camptothecin and Topotecan at Normal and Acidic pH. Cancer Res. 57, 4811-4816, (1997); Teicher, B. A., Holden, S. A., Khandakar, V., Herman, T. S. Addition of a topoisomerase I inhibitor to trimodality therapy [cis-diamminedichloroplatinum(II)/heat/radiation] in a murine tumor. J. Cancer Res. Clinical Oncol. 119, 645-651, (1993); Wood, P. J., Sansom, J. M., Newell, K., Tannock, I. F., Stratford, I. J. Reduction of tumour intracellular pH and enhancement of melphalan cytotoxicity by the ionophore nigericin. Int. J. Cancer, 60, 264-268. (1995); Vukovic, V., Tannock, I. F. Influence of low pH on cytotoxicity of paclitaxel, mitoxantrone and topotecan. Brit. J. Cancer 75, 1167-1172, (1997); Wachsberger, P. R., Burd, R., Wahl, M. L., Leeper, D. B. Betulinic acid sensitization of of low pH adapted human melanoma cells to hyperthermia. Int. J. Hyperthermia 18, 153-164, (2002); and Hoffner, J., Schottelius, J., Feichtinger, D., Chen, P. Chemistry of the 2,5-didehydropyridine biradical: computational, kinetic, and trapping studies toward drug design. J. Am. Chem. Soc. 120, 376-385, (1998). Hyperglycemia and/or such drugs as amiloride, nigericin, and hydralyzine, are able to lower the intracellular pH of cancer cells as well. At dosages that do not affect the normal cells, amiloride and nigericin has been reported to drop the intracellular pH in a number of tumor cell types from 7.2 to 6.2-6.6. See, e.g., Adams, G. E., Stratford, I. J. Bioreductive drugs for cancer therapy: The search for tumor specificity. Int. J. Radiat. Oncol. Biol. Phys. 29, 231-238, (1994); Priyadarsini, K. I., Dennis, M. F., Naylor, M. A., Stratford, M. R. L., Wardman, P. Free Radical Intermediates in the Reduction of Quinoxaline N-Oxide Antitumor Drugs: Redox and Prototropic Reactions. J. Am. Chem. Soc. 118, 5648-5654, (1996). and references therein; Stubbs, M., Rodrigues, L., Howe, F. A., Wang, J. Jeong, K. S., Veech, R. L., Griffiths, J. R. Cancer Res. Metabolic Consequences of a Reversed pH Gradient in Rat Tumors. 54, 4011-4016, (1994); Lyons, J. C., Ross, B. D. Song, C. W. Enhancement of hyperthermia effect in vivo by amiloride and dids. Int. J. Radiat. Oncol. Biol. Phys. 25, 95-103, (1993); Song, C. W., Lyons, J. C., Griffin, R. J., Makepeace, C. M. Thermosensitization by lowering intracellular pH with 5-(N-ethyl-N-isopropyl) amiloride. Radiother. Oncol. 27, 252-258, (1993); Song, C. W., Lyons, J. C., Griffin, R. J., Makepeace, C. M., Cragoe, E. J., Jr. Increase in Thermosensitivity of Tumor Cells by Lowering Intracellular pH. Cancer Res. 53, 1599-1601, (1993); Song, C. W., Kim, G. E., Lyons, J. C., Makepeace, C. M., Griffin, R. J., Rao, G. H., Cragoe, E. J. Jr. Thermosensitization by increasing intracellular acidity with amiloride and its analogs. Int. J. Radiat. Oncol. Biol. Phys. 30, 1161-1169, (1994); Lyons, J. C., Song, C. Killing cf Hypoxic Cells by lowering the Intracellular pH in Combination with Hyperthermia. Radiat. Res. 141, 216-218, (1995); and Newell, K., Wood, P., Stratford, I., Tannock, I. Effects of agents which inhibit the regulation of intracellular pH on murine solid tumours. Br. J. Cancer 66, 311-317, (1992). When combined with hyperglycemia and/or hypoxia, further acidification to pH as low as 5.5 is possible. See, e.g., Osinsky, S. P., Levitin, I. Y., Bubnovskaya, L. N., Ganusevich, II., Sigan, A. L., Tsykalova, M. V., Zagorujko, L. I. Exp. Oncol. 21, 216, (1999); Tannock, I. F., Rotin, D. Acid pH in Tumors and Its Potential for Therapeutic Exploitation. Cancer Res. 49, 4373-4384, (1989); Wike-Hooley, J. L., Haveman, J., Reinhold, H. S. The relevance of tumour pH to the treatment of malignant disease. Radiother Oncol. 2, 343-366, (1984).
One way to take advantage of these differences involves the development of pH-gated DNA cleaving agents. See Kar, M., Basak, A. Chem. Rev. Design, Synthesis, and Biological Activity of Unnatural Enediynes and Related Analogues Equipped with pH-Dependent or Phototriggering Devices. 107, 2861-2890, (2007). The promise of DNA as a target for cancer therapy is illustrated by the astounding biological activity of natural enediyne antibiotics. See Stanulla, M., Wang, J., Cnervinsy, D. S., Thandla, S , Aplan, P. D., DNA cleavage within the MLL breakpoint cluster region is a specific event which occurs as part of higher-order chromatin fragmentation during the initial stages of apoptosis. Mol. Cell. Biol., 17, 4070-4079, (1997). These compounds, hailed as “the most potent family of anticancer agents,” can induce double strand DNA cleavage via abstraction of two hydrogen atoms, one from each strand of DNA duplex, with the most efficient double strand DNA-cleaver from this family, calicheamicin, forming ˜25-33% of double strand breaks. See Galm, U., Hager, M. H., Van Lanen, S. G., Ju, J., Thorson, J. S., Shen, B. Antitumor Antibiotics: Bleomycin, Enediynes, and Mitomycin. Chem. Rev. 105, 739-758, (2005) and Elmroth, K., Nygren, J., Martensson, S., Ismail, I. H., Hammarsten, O. Cleavage of Cellular DNA by Calicheamicin γ1. DNA Repair 2, 363-374, (2003). While single strand (ss) DNA damage is easily repaired by enzymatic processes, the repair of double strand (ds) DNA cleavage is more difficult and, thus, it can initiate self-programmed cell death, or apoptosis. See Watson, J. D., Baker, T. A., Bell, S. P., Gann, A., Levine, M., Losick, R. (2004). Molecular Biology of the Gene, ch. 9 and 10. Peason Benjamin Cummings, CSHL Press. 5th ed. Therefore, ds DNA cleavage is a more efficient tool for the cancer therapy as long as it can be induced selectively in cancer cells avoiding damage to healthy cells.
Light-activated DNA-cleavers provide spatial and temporal control over DNA cleavage, allowing drug activation in the right place and at the right time, when concentration of the drug is highest in the cancer tissues. See Armitage, B. Photocleavage of Nucleic Acids. Chem. Rev. 98, 1171-1200, (1998). In a previous work, enhanced selectivity was achieved via the development of the first pH-controlled system for ds DNA cleavage. This hybrid system combined an efficient DNA-cleaver capable of operating within the physiological pH range when attached to a pH-sensitive functionality. See Kovalenko, S. V., Alabugin, I. V. Lysine-enediyne conjugates as photochemically triggered DNA double-strand cleavage agents. Chem. Comm. 1444-1446, (2005) and Yang, W.-Y., Breiner, B., Kovalenko, S. V., Ben, C., Singh, M., LeGrand, S. N., Sang, Q.-X. A., Strouse, G. F., Copland, J. A., Alabugin, I. V. C-Lysine Conjugates: pH-Controlled Light-Activated Reagents for Efficient Double-Stranded DNA Cleave with Implications for Cancer Therapy. J. Am. Chem. Soc. 131, 11458-11470, (2009).
The new family of pH-dependent DNA photocleavers displayed a number of unique properties. In particular, these lysine conjugates showed efficient ds DNA cleavage (ss:ds=2:1) rivaling the efficiency of calicheamicin, selective cleavage at G-sites flanking AT-tracks and ability to convert ss DNA damage into ds DNA damage. See Breiner, B., Schlatterer, J. C., Kovalenko, S. V., Greenvaum, N. L., Alabugin, I. V. Protected 32P-labels in Deoxyribonucleotides: Investigation of Sequence Selectivity of DNA Photocleavage by Enediyne-, Fulvene-, and Acetylene-lysine Conjugates. Angew. Chem. Int. Ed. 45, 3666-3670, (2007) and Breiner, B., Schlatterer, J. C., Kovalenko, S. V., Greenbaum, N. L., Alabugin, I. V. DNA Damage-Site Recognition by Lysine Conjugates. Proc. Natl. Acad. Sci. U.S.A., 104, 13016-13021, (2007). It was also shown that these compounds cleave intracellular DNA, display light-induced cytotoxicity to several cancer cell lines and are susceptible to two-photon absorption (TPA) activation. See Yang, W.-Y., Cao, Q., Callahan, C., Galvis, C., Sang, A. Q.-X., Alabugin, I. V. Intracellular DNA damage by lysine-acetylene conjugates. J. of Nucleic Acids Article ID 931394, (2010) and Kauffman, J. F., Turner, J. M., Alabugin, I. V., Breiner, B., Kovalenko, S. V., Badaeva, E. A., Masunov A., Tretiak, S. J. Phys. Chem. A. 110, 241-251, (2006).